The present invention relates to circuits for connecting the output terminals of a signal source across a load impedance.
The operation of many electrical and electronic systems relies on the application of a voltage across a load from a voltage source and it is recognized in the art that the performance of such systems depends on the quality of the coupling between the source and the load, i.e. the extent to which the voltage across the load corresponds to, or equals, the open circuit source voltage or an amplified version of the source voltage.
Ideal coupling would result in the appearance of a voltage, or possibly a current, exactly proportional to the open circuit source voltage across the load with respect to both time and frequency domain characteristics as the source signal.
The achievement of ideal coupling would be advantageous in many electronics systems. For example, if the signal source is an electronic or electrical circuit port, the load is an oscilloscope employed to monitor the time domain waveform of the voltage developed by the signal source and the intervening coupling medium is the oscilloscope probe and associated cabling, the oscilloscope trace will accurately reflect the waveform of the source voltage only if the voltage applied to the oscilloscope inputs is identical to the open circuit source voltage.
For many other types of electronics purposes, ideal coupling would produce optimum results. This would also be the case in a variety of audio, data transmission, communication and telecommunication systems. However, there is no known circuit arrangement which achieves perfect transfer, or ideal coupling, of the source voltage to the load.
Known coupling circuits fail to achieve ideal coupling for several reasons. Firstly, the voltage source is connected to the load via a coupling medium which has a finite impedance at any given frequency, and which will therefore be responsible for a voltage drop. Secondly, a practical voltage source behaves like an ideal voltage source in series with a source impedance. Therefore, when current is drawn from such a source, there is a voltage drop across the source resistance.
It is known to minimize source-to-load voltage drops, or signal attenuation, by the provision of an active circuit, known as an amplifier stage, between the source and load circuits. When such a circuit is used, for example when an operational amplifier is used as a unity gain voltage follower, the ratio of load voltage, V.sub.L, to source voltage, V.sub.i, can be expressed as follows: ##EQU1## Where A.sub.V is the open circuit voltage gain of the amplifier stage, R.sub.IN is the driving point input resistance of the amplifier stage,
R.sub.OUT is the driving point output resistance of the amplifier stage,
Z.sub.i is the source impedance, and
Z.sub.L is the load impedance.
If R.sub.IN is much greater than the absolute value of Z.sub.i the absolute value of Z.sub.L is much greater than R.sub.OUT, and A.sub.V is approximately equal to 1, the load-to-source voltage ratio will be almost equal to unity. An ideal amplifier will have a value for R.sub.IN approaching infinity, a value for R.sub.OUT approaching zero and a value for A.sub.V substantially equal to unity.
However, practical amplifiers, when connected as a unity gain voltage follower, or amplifier, which include bipolar emitter followers and MOSFET source followers, have characteristics which are far from ideal. Emitter followers typically establish driving point input resistances that are rarely larger than a few hundred k.omega. and driving point output resistances that are rarely smaller than several tens of .omega.. Additionally, their open circuit voltage gains are usually no better than 0.95. MOSFET source followers provide a reasonable approximation of an infinitely large driving point input resistance, but their output resistance can be of the order of 100 .omega.. Moreover, the low frequency open circuit voltage gain of a MOSFET source follower can be as low as 0.75 and when compared with bipolar emitter followers, the frequency response of a MOSFET source follower is substantially inferior. Both of these types of followers have marginal high frequency response capabilities. In the case of an emitter follower circuit, the frequency response can also be significantly underdamped, which would promote circuit and system instability, particularly when the load is highly reactive.
When a signal source is coupled to a load having an impedance with a reactive component, the current through the load is out of phase with the voltage across the load. Conventional coupling devices, including conventional amplifiers and impedance buffers, can not supply the correct out-of-phase load current. As a result, when the load is reactive or has a reactive component, even the best coupling devices can not avoid distorting the signal across the load. Such distortion is particularly apparent during those portions of the signal cycle when the load current polarity is opposite to the load voltage polarity. Those in the art will recognize that the magnitude of the reactive component of a load impedance which will be undesirable is that which causes a load voltage distortion that measurably deteriorates the performance of the system in which the circuit is installed.
In addition, all coupling devices have some reactances, which may be undesired parasitic reactances, which will, in the prior art, unavoidably produce a time delay between the source voltage and the load voltage, as well as phase shifts within the coupling device which can lead to load voltage distortion.